The invention relates to an impedance matching circuit and, more particularly, to an impedance matching circuit which is suitable to be applied on an IC chip for adjusting impedance variation caused by the transistors of the impedance matching circuit.
In a cable communicating system, impedance matching circuit is necessary to be implemented in the cabling interface of a communicating device for reducing or eliminating signal reflections caused by unmatched impedances between the cable and the circuits inside the communicating device.
FIG. 1 is a schematic drawing illustrating the basic principle of the impedance matching of prior art. As shown, the output device 1 outputs a signal to drive a load unit 2 via a cable 3. In order to accomplish the object of impedance matching, the characteristic impedance of the cable has to match the impedances of the inside circuits of both the output device 1 and the load unit 2. Otherwise, signal reflections may occur. For example, if the output resistor 10 of the output device 1 is 50 xcexa9, then the impedance of the cable 3 must be 50 xcexa9, and the input resistor 20 of the load unit 2 must be 50 xcexa9 as well.
Generally, the values of resistances of the circuits built-in an IC (Integrated Circuit) chip are variable due to the inevitable deviations of semiconductor manufacturing processes. It is relatively difficult to precisely control the resistance of the IC chip for the purpose of impedance matching. Therefore, the conventional way is to apply a precision resistor outside the IC chip to achieve the object of impedance matching. However, the drawback of this prior art technique is the increases of the complexity of circuit design, the usage of electronic components, and the cost.
Because it is relatively difficult to design a linear resistor (for example, by using poly N-wells) with precise resistance inside an IC chip, another conventional way is to apply an adjustable resistor circuit inside the IC chip to compensate the variation of the resistance of the IC chip caused by semiconductor manufacturing processes.
FIG. 2 is a schematic diagram showing an on-chip impedance matching circuit of prior art. As shown in FIG. 2, the impedance matching circuit 4 includes a plurality of MOS transistors 40, 41, 42. These MOS transistors 40, 41, 42 are controlled by providing switching voltages to the gates S1, S2, S3 respectively so as to determinate whether these transistors are switched on or not. For example, if the gate S2 is provided with a switching voltage, the MOS transistor 41 will be switched on, and the resistance value of the output resistor (Rout) will be equal to the sum of the values of the resistors from the resistor 44 through the resistor 45. However, there is still a serious problem in such design.
FIG. 3 is a schematic diagram showing the circuits of the interior resistors 400, 410, 420 of the MOS transistors 40, 41, 42 of FIG. 2. As previously described in FIG. 2, when one of the MOS transistors 40, 41, 42 is switched on, then a resistance will also be generated inside that MOS transistor. These inevitable resistances caused by switching on these MOS transistors 40, 41, 42 can be schematically represented by the interior resistors 400, 410, 420 respectively. That is, When the MOS transistor 41 is switched on, i.e. S2 is short, the output resistor value (Rout) is actually equal to the sum of resistors 44 through 45, and 410. Therefore, when designing an on-chip impedance matching circuit, not only the resistors 43, 44, 45 should be calculated, but also the interior resistors 400, 410, 420 must be considered. Moreover, the values of such interior resistors 400, 410, 420 are variable by applying different values of input voltage Vi, which will distort the output signals and impede the functions of the IC chip.
The first object of the present invention is to provide an impedance matching circuit which is suitable to be applied on an IC chip for adjusting impedance variation of the transistors formed on the IC chip.
The second object of the present invention is to provide an impedance matching circuit which comprises a resistor unit and an operational amplifier. By utilizing the ideal characteristic of the operational amplifier, the variations of the interior resistors caused by the transistors can be eliminated, and thus the resistance of the impedance matching circuit can be precisely controlled and selected.
In an embodiment of the impedance matching circuit in accordance with the present invention, the impedance matching circuit comprises a resistor unit, an OP amplifier circuit connected with the resistor unit, a feedback selecting circuit connected in parallel with the OP amplifier circuit, and a resistor selecting circuit connected with both the OP amplifier circuit and the feedback selecting circuit. The feedback selecting circuit further includes a plurality of switching circuits for enabling some of the resistors furnished in the resistor selecting circuit. By selecting and actuating one of the switching circuits, some certain resistors will be enabled so as to adjust the resistance value of the resistor selecting circuit. The resistor unit and the switching circuits are designed in such a manner that the resistor unit is able to compensate an equivalent resistance of the switching circuit which is actuated. As a result, the impedance of the whole impedance matching circuit is precisely matching with the adjusted resistance value of the resistor selecting circuit and will not be influenced by any variation of the input signals.
Other and her features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.